This invention generally relates to an improved method of measuring analog voltages through isolated circuits and an apparatus for the same.
When measuring an analog input voltage through an isolated circuit, it is a practice to use an isolation transformer (hereinafter referred to as transformer). For measurement of a plurality of analog input voltages, the voltages applied across primary windings of transformers are selected on the time division base. In the case of transmitting a voltage through the transformer, even when a load or a resistor connected across the primary winding of the transformer is infinite and coupling coefficient of the transformer is approximately 1 (one), a ratio between the primary voltage and the secondary voltage cannot be identical with the winding ratio of transformer because of influence of sag. "Sag" refers to that phenomenon which occurs when an analog input voltage is applied across the primary winding of transformer, a secondary voltage is decreased with time with the result that the analog input voltage is prevented from being transmitted accurately to the secondary side. Sag is caused by the fact that both direct current resistances of the primary winding of transformer and of an input selection switch connected across the primary winding of the transformer are of finite value, and the magnitude of sag is given by the following equation ##EQU2## WHERE V.sub.2 represents a secondary voltage of the transformer, V.sub.in an analog input voltage, L a primary inductance of the transformer, R a sum of direct current resistances of the primary winding and the input selection switch, and t time lapse from closure of the input selection switch.
Compensation for influence of sag is needed for a highly accurate measurement. The need for such compensation is commonly recognized and a typical measure is with the use of a three winding type transformer.
According to the expedient mentioned above, voltages developing across secondary windings connected to a common component of a measuring instrument are integrated and the integrated outputs are added to the output of an amplifier to make equal the integrated outputs to reductions due to sag, thereby compensating for the influence of sag.
The prior art measure had difficulties with accurate compensation for sag for the following reasons (1) and (2).
(1) Input selection switches cannot have an identical direct current resistance because of the irregularities of their elements, and
(2) Transformers cannot have an identical inductance because of the irregularities of their elements. (In general, about .+-.3 % irregularities are allowed.) It will be seen from the above reasons (1) and (2) and the above-mentioned equation for the voltage V.sub.2 that the magnitude of sag varies from input point to input point.
On the other hand, the integrated output for compensating for sag is smaller than for larger sag. Therefore, in view of the above-mentioned reasons (1) and (2), it is difficult to compensate for sag with accuracy.
In order to accurately compensate for the influence of sag, it is considered that, in connection with the reason (1), the input selection switch is connected in series with a variable resistor and the variable resistor is so adjusted that a sum of direct current resistances of the input selection switch and variable resistor of each channel are unified to a specified value and that, in connection with the reason (2), a transformer having a constant inductance is strictly selected for each channel. However, even if the above points are considered, variation in the ambient temperature causes the total resistance R and the inductance L in the above-mentioned equation for the voltage V.sub.2 to vary and as a result, it is difficult to accurately compensate for sag without fail. In addition, realization of the aforementioned points inevitably leads to a high cost and the adjustment of the variable resistor is time consuming.
In conclusion, the prior art technique hereinbefore described has disadvantages as follows.
(a) Failure to compensate for sag with high accuracy (variation in ambient temperature involved);
(b) High cost;
(c) Time consuming adjustment work; and
(d) Difficulty of characteristic control of the parts.